The electrical parameters of ideal devices, such as theoretical capacitors and resistors are invariant with respect to the impressed voltage or the current passing through the devices and the length of time these voltages or currents are applied. Unfortunately, many devices do not behave in an ideal manner. For one such class of device for example, the equivalent capacitance C or conductance G will vary in time when a pulse of voltage or current is applied.
An apparatus to measure these time variant parameters, C-t and G-t, is often used for determining the characteristics of semiconductors such as diodes so as to calculate carrier lifetimes, trap levels, and the like.
Apparatuses for performing the C-t and G-t measurement are known in the prior art, however, in the past it has not been possible to accurately make these measurements. There are several sources of this measurement inaccuracy. First, a filter has been employed in the circuitry which causes internal time delays so that the admittance value (j.omega.C+G) is actually measured after the passage of a given period from the point of occurrence of the actual change in admittance. Second, the accuracy of the circuits used for converting the current proportional to the admittance value to a voltage has been too low to insure low measurement error. Finally, in the prior art the analog to digital converters (A/D) used for converting an analog voltage corresponding to the admittance value to a digital value has been affected by the actual amplitude of the measuring signal. Thus, a change in the measuring level causes additional measurement error.